Cooling fan modules are used to cool the engine in motor vehicles. In this connection, it is the aim to improve the cooling performance for the engine, transmission and the comfort of the vehicle occupants, especially with regard to increasing fan efficiency and minimising the noise generated by the cooling fan module.
Generally a cooling fan module consists of a fan impeller, a motor located at the centre of the fan to drive the fan impeller, and a frame or shroud which comprises assembly struts for fixing the motor. Further, the fan impeller of a cooling fan module is designed to produce an air flow with which the heat generated by the engine is removed.
In an engine cooling fan module, pressure downstream of or after the fan is higher than pressure upstream of or in front of the fan for the effective fan operating range. This pressure difference drives airflow from downstream of the fan back to upstream of it forming undesired recirculating flow through the running clearance between the frame orifice or shroud orifice and the fan ring of the fan impeller. Due to the rotation of the fan, there is a swirling motion in downstream of the fan, this swirling motion is carried to front of the fan by this recirculating flow, friction force from the rotating fan ring also contributes to this swirling motion in the recirculating flow. This recirculating flow then is drawn back into the fan again in fan blade tip region. As a result, in the tip region of the fan blade there is a large variation of tangential airflow velocity, and the blade is at varying angles of attack different from that of the main flow, which leads to airflow separation in blade tip region, this portion of blade becomes low in efficiency, noisy and inconsistent with the rest of the fan blade. At the largest radius, the tip region of the fan blade has the largest working potential and biggest influence on the performance of the entire fan. This reduced performance at the blade tip region decreases efficiency of the fan module significantly.